[PATCH 08/12] drm/sun4i: sun6i_mipi_dsi: Refactor vertical video start delay

[Jagan Teki <jagan@amarulasolutions.com>]

Accordingly to BPI-M64-bsp DE DSI code Video start delay
can be computed by subtracting total vertical timing with
front porch timing and with adding 1 delay line for TCON.

This patch simply add the start_delay logic from BPI-M64-bsp,
w/o this new computation, the DSI on A64 encounter vblank time out.

[CRTC:36:crtc-0] vblank wait timed out

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
---
 drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
index 9918fdb990ff..217db74c6dc3 100644
--- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
+++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
@@ -358,7 +358,17 @@ static void sun6i_dsi_inst_init(struct sun6i_dsi *dsi,
 static u16 sun6i_dsi_get_video_start_delay(struct sun6i_dsi *dsi,
 					   struct drm_display_mode *mode)
 {
-	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + 1;
+	u32 vfp = mode->vsync_start - mode->vdisplay;
+	u32 start_delay;
+
+	start_delay = mode->vtotal - vfp + 1;
+	if (start_delay > mode->vtotal)
+		start_delay -= mode->vtotal;
+
+	if (!start_delay)
+		start_delay = 1;
+
+	return start_delay;
 }
 
 static void sun6i_dsi_setup_burst(struct sun6i_dsi *dsi,
-- 
2.18.0.321.gffc6fa0e3